Means and method of grinding thread cutting tools



F. SCHIFANDO April. 9, 1957 MEANS AND METHOD OF GRINDING THREAD CUTTINGTOOLS Filed NOV. 2 1951 5 Sheets-Sheet 1 Sn w QM W \A wwt 3 QM .wm Mm Q3 Q A i! w I 2 SM n llll l 41 T {.5 4 8 m g R. N. Y K N mm INVENTOR.Fred Sch/7471140 April 9, 1957 F. SCHIFANDO 2,787,868

MEANS AND METHOD OF GRINDING THREAD cu'rmc TOOLS Filed Nov. 2, 1951 5Sheets-Sheet 2 c \HUI xii 12 F/gz INVENTOR. Fred Jc/wfimdo ATTORNEYApril 9, 1957 F. SCHIFANDO 2,787,868

MEANSAND METHOD OF GRINDING THREAD CUTTING TOOLS Filed Nov. 2, 1951 5She ets-Sheet 3 INVENTOR.

- Fred $C/7/f0/7Q'0 ATTORNEY April 9, 1957 F. SQHIFANDO 2,787,868

MEANS AND METHOD OF GRINDING THREAD CUTTING TOOLS Filed Nov. 2, 1951 5Sheets-Sheet 4 a? n2 a4 86 80 IN VEN TOR.

' m0 [50 c 55d Fred scmfa/zao ZED/WM AT TORNEY April 9, 1957 F.SCHIFANDO 2, 8 ,8

MEANS AND METHOD OF GRINDING THREAD CUTTING TOOLS Filed Nov. 2, 1951 5Sheets-Sheet 5 INVENTOR. Fred sm/fafld 4 ATTORNEY ,only two of itslands.

United States Patent 6 MEANS AND METHOD OF GRINDING THREAD CUTTING TOOLSFred Schifando, New York, N. Y.

Application November 2, 1951, Serial No. 255,757

11 Claims. (CI. 51-96) This invention relates to means and the method ofgrinding thread cutting tools such as taps, dies and chasers.

The leading ends of the lands on conventional taps,-

dies and chasers are provided with chamfers extending lengthwise of thelands. These chamfers are produced by eccentrically grinding each landof the tool to thus form a relief or clearance extending from thecutting edge to the heel of the land.

Threads cut by the conventional threading tools are frequentlydefective. Some of the defects are strip threads, taper threads,eccentric threads and errors in the lead of the threads.

The object of the present invention is to produce threading tools whichare ground in a novel manner, by eccentric grinding; and by novel meansto avoid the above mentioned defects.

To this end the present invention provides means whereby each land ofthe threading tool is moved simultaneously in two directions, relativelyto the grinding wheel, namely, crosswise and slightly lengthwise of theland, so that the resultant path of grinding of the various threadcrests in the chamfer is in the same direction as the angles of thethread teeth cross-wise of the land.

In carrying out the invention provision is made for supporting thethreading tool on a fixture relatively to the grinding wheel and thengrinding the various lands in- Other features and advantages willhereinafter appear. 7

In the drawing which forms part of the specification.

Fig. 1 is a side elevation, partly in section, of a novel machine forgrinding a chamfer on a thread cutting tool such as a tap;

Fig. 2 is a view, in longitudinal section, of one of various guideswhich may be used in the machine of Fig. 1;

Fig. 3 is a view, on a larger scale, of the forward end of the tap shownin Fig. 1;

Fig. 4 is a cross-section of the tap taken on the line 44 of Fig. 3looking in the direction of the arrows at said line;

Fig. 5 is a developed view of the tap shown in Fig. 3 showing thefourlands with respect to each other. The view also shows the shapes ofchamfer faces on the threads included inthe chamfer of each land;

Fig. 6 is a view in section, similar to Fig. 4, but of a conventionaltap, illustrating the relief on the threads included in its chamfer;

Fig. 7 is a developed view, similar to Fig. 5, of a. portion of theconventional tap illustrated in Fig. 6 but shows It also shows theshapes of the chamfer faces on threads included in the chamfer of eachland;

Fig. 8 is a side elevation, partly in section, of a machine for grindingof the chamfer on a chaser;

Cir

ice

Fig. 12 shows the upper left hand corner, on a larger scale, of thechaser shown in Fig. 10. Similar portionsof the other three chasers of.Fig. 9 are shown behind it for the purpose of comparison with it andeach other;

Fig. 13 is a view showing the shapes of the chamfer faces on the leadingthreads of a conventional chaser; and

Fig. 14 an elevation showing the grinding wheel and its support.

The machine 15 illustrated in Fig. 1 includes a fixture 16 for grindinga chamfer 18 on the thread cutting teeth 20 at the leading end of eachland 22, 22a, 22b and 220 of a tap 24. The lands are formed by flutes 25which extend axially of the tap. The fixture 16 includes a base plate 26to which are secured brackets 28 and 34 the object of the bracket 30being described later. bracket 28 has a housing or hub portion 32 havinga horizontal bore 34 in which is located a gmide as in the form of abushing. The guide 36 has a slot 38 in its upper portion into which thelower, reduced end 40 of a thumb screw 42 extends. The thumb screw 42 isthreaded through the upper portion of the housing 32 and may be screweddown against the guide 36 to prevent accidental axial displacement ofthe guide. The screw 42 also prevents circumferential displacement ofthe guide 36, because its end 40 engages in the slot 38. It will beunderstood that after backing off the screw 42 the guide 36 may beadjusted lengthwise in the housing bore 34.

The guide 36 has an axial bore :4 into which the tap 24 may be insertedfrom the right hand end. The guide 36 has, at the left hand end of itsbore 44 screw threads 46 with which the tap threads engage to guide thetap 24.

After assembling the tap 24 and guide 36 the latter may be inserted intothe housing 32 from its left hand end.

A grinding wheel 47, is located at the leading end of v The grindingwheel 47 is normally 7 the tap 24 (Fig. 1). clear of the tap and ismounted to rotate on a suitable bracket 47a (Fig. 14), which bracket maybe pivotally I supported at 47b on the base plate 26 so that the wheel47 may be moved bodily toward and away from the tap 24. The wheel 47 hasa grinding face 48 which is angularly disposed with reference to the tap24 so as to grind the chamfer 18 on the thread teeth at the leading endof each tap land.

After the tap 24 is inserted into the fixture 16 it is advanced towardsthe grinding wheel, which in the meantime has been lowered into grindingposition. The tap is then positioned about its axis for grinding. Forthis purpose there is provided a stop pin 52 supported for verticalmovement in :1 lug 54 on housing 32. The pin 52 is pushed downwardly toproject its lower end into one of the tap flutes 25. The tap is thenrotated until the land 22 engages the stop pin 52 thus causing the land22 to extend upwardly towards the grinding wheel. The tap with its guideor bushing 36 is then advanced axially until the first thread justtouches the rotating grinding wheel. This position may be observed bysparks coming from the work. The thumb screw 42 is then screwed down tosecure the guide 36 to the fixture 16.

After the guide 36 has been so secured an end stop 56 supported in theupper end of the fixed bracket 30 is moved into engagement with theright hand end or shank 58 of the tap 24. The end stop 56 is then lockedin this position by a set screw 69. The pin 52 is subse- The quentlyraised clear of the land 22 where it may be held temporarily by a springpressed ball 61 in the lug 54 and adapted to engage in an annular groove62 in pin 52. The tap 24 is then rotated back-and-forth in its guide 36relatively to the grinding wheel, from the cutting edge 63 to the heel64- (Figs. 3, 4 and 5) of the land 22, to grind the chamfer 18.

To rotate the tap back-and-forth there is provided a knurled fingerwheel 62:: demountably secured to the shank or square end 58 of the tap24 by a set screw 65. Thus it will be understood that by rotating thefingerwheel 62a back and-forth the tap is rotated about its axis in thefixed guide 36 and relatively to the wheel 47. During the back-and-forthrotation transversely of the land, the tap is caused to moveback-and-forth axiall i of the guide by the thread 46 and relatively .tothe grinding wheel 47.

During the grinding operation, the grinding wheel 47 may be moved insmall increments towards the tap, by a thumb screw 47c (Fig. 14), untilthe desired depth of chamfer has been reached.

Movement of the wheel 47 towards the tap may be limited or stopped inits extreme position by suitable means including a stop 47d. The stop47d may engage the bracket 47:! which supports the grinding wheel.

After the chamfer has been ground on the first land 22 the grindingwheel 47 is raised slightly by the screw 47c and the tan is then rotatedabout its axis until the land 22 passes the stop pin 52. The latter isthen lowered into the path of the next land 22a in order to position thelatter at the grinding wheel. During the rotation of the tap, to bringthe second land 22a into engagement with the stop pin 52, the tip isadvanced axially a slight distance due to the threads 46 in the guide36. This caused the shank 58 of the tap to move away from the end stop56. Before proceeding to grind the chamfer on the second land 22a thetap is repositioned, axially, against the end stop 56. This is elfectedby first backing off the thumb screw 4-2 and then pushing the tap withits guide 36 rightwnrdly (Fig. 1) until the shank 58 of the tap engagesthe end stop 55. The thumb screw 42 is then screwed down to secure theguide bushing 36. The stop pin 52 may then be raised out of engagementwith the land 22a so that the tap 24 may be freely rotated backand-forthby the finger wheel 62a to grind the chamfer 18 on the land 22a. Thegrinding wheel 47 is then lowered by the screw 47c to grind the chamferon the land 22a.

The above describedoperations are repeated for grinding the chamfer oneach of the lands 22b and 220 and include the repositioning of the tapagainst the end stop 56 before each grinding operation. Because of therepositioning operation of the tap against the end stop 56 and limitingthe movements of the grinding wheel toward the tap the chamfers on allof the lands 22, 22a, 22b and 22c are ground to the same depth. In otherwords the chamfer on all of the lands will be the same distance,radially, from the axis of the tap.

Provision is made for handling taps of various sizes, smaller or largerthan the one 245, shown in Fig. 1. To this end other bushing guides areused in the housing 32. which have the same outside diameter as theguide 36 to fit into the bore 34- of housing 32, but in which the bore44 varies to fit the diameter of the larger or smaller tap. A guide 36afor a tap of larger diameter is shown in Fig. 2. This guide 36o fits thebore 34 of housing 32. The guide 36:! is similarly provided with a bore44a which is larger in diameter than the one, 44, shown in Fig. l. Theguide 36:: has thread 46a at its left hand end which is the same size asthat of the larger tap. The guide 36:1 is also provided with a slot 38ain cooperation with the end 40 of the lock screw 42.

The chamfers 18 on the various lands after grinding 1 width from thecutting edge to the heel of the land.

appear as in Fig. 3 where the crowns of the leading thread sections orteeth on the :four lands have been re moved thus leaving the chamferfaces 66 011 the leading thread cutting teeth. This is the result ofconcentric grinding since the tap 24 is rotated back-and-forth about itsaxis from the cutting edge 63 to the heel 64 of the land during thechamfer grinding operation. These chamfer faces extend in the samedirection as the thread cutting teeth. Each complete chamfer face is ofuniform Fig. 5 shows the chamfer faces 66 on all of the lands. It showshow the two sides of all of the complete chamfcr faces 66 are of uniformwidth from the cutting edge to the heel of each land 'thus making theside edges of each complete face 66 parallel and of smaller width on theteeth of each land from the leading end of the tap inward. The faces ofthe various sections of the same thread as it extends over the fourlands are also of smaller width from top to bottom of Fig. 5.

For the purpose of comparison the conventional tap 24x is illustrated inFigs. 6 and '7 where parts similar to those of .Fig. 3 have the samereference numerals with the subscript x added. In the tap 24x eachthread tooth included in the chamfer has a relief 74 behind the cutingedge 63x of each land. This relief is usually pro duced by mechanismincluding a cam which causes the grinding wheel to move towards the axisof the tap MA: as the grinding proceeds from the cutting edge to theheel of the land. Grinding the relief 74 produces faces, on the threadcutting teeth, with sides which are not parallel but are inclinedtowards each other. The present invention, however, providescomparatively simple means to grind the chamfer because of the methodemployed,

namely that of concentric grinding in the direction of the threadcrosswise of the lands.

Another machine embodying the invention is shown in Fig. 8. This machineis used for grinding chamfer faces 82 on the thread cutting teeth 84 atleading ends of the land 86 of a chaser 88 which is one of a pluralityof chasers used to cut external threads. Four of these chasers, 88, 88a,88b and 88c are shown in Fig. 9 where they are arranged to cut anexternal thread 91 on a bolt or conduit .92. The machine 80 is similar,in many respects, to the machine 15 of Fig. 1 and includes a base plate94 on which is rigidly supported a bracket 96. The bracket 96 .isprovided with a housing or hub portion 98 having a horizontal bore 10!)in which is supported a guide, in the form of a shaft 102.

The guide .102, like the guide 36 of Fig. '1, also has a slot 104 in itsupper portion and extends axially thereof. The lower end 106 of a thumbscrew 108 engages in the slot 104 to prevent rotation of the guide 102about its axis and to secure said guide axially of the housing 98 at'will. The guide 102 has, on its left hand end, an external thread 110which thread is the same size as an internal thread 112 of the chaser83.

A chaser supporting element 114 includes a hub 116 by which it isrotata'bly supported on a bearing portion 118 of the guide shaft orthreaded element 1 32. The supporting element 114 also has a knurledfinger piece 120 by which it may be rotated back-and-forth about theguide 102. A lug 122 extending forwardly from the face of the fingerwheel 120 is provided with a vertical shoulder 126. The chaser ispositioned on the element 114 by placing it against the shoulder 126 andagainst a wall portion 128 of the lug 122. The chaser is detachablysecured to the lug 122 by a screw 130 which passes through a washer 132,a hole 134 (Fig. 10) in the chaser, the screw being threaded into a hole136 in the wall 128 of the lug 122. A grinding wheel 138 is supported bymeans which may be similar to that shown in Fig. 14 in connection withthe Wheel 47.

To assemble the guide 102, chaser supporting element 114 and chaser 88on the bracket 96, the wheel is moved up to a position above that shownin Fig. 8. The guide 102 may then be placed into the bore 100 of housing98 from the right hand side of the bracket '96 with its bean ing portion118 extendingheyond the left end of the housing 98 with the screw end106 in the slot 104. The chaser supporting element 114 may next beplaced on the bearing portion 118 of guide 102. The guide 102 is thenpositioned axially so that a few teeth, of thread 110, at its leadingend are to the left of a vertical plane in which the face 126 of lug 122is located. The chaser 88 is then placed on the lug 122 and the chaserthread cutting teeth 112 are caused to engage the thread 110 of theguide 102, the pitch of the thread 110 and teeth 112 being the same. Ifthe thread teeth of chaser 88 and the thread of the guide 102 do notregister properly an adjustment may be made by sliding the guide 102leftwardly or rightwardly in its housing 98 until they do register.After the threads engage properly the chaser 88 may be secured to thelug 122 by the screw 132 as previously described.

After thus assembling the chaser 88, support 114 and guide 102, theentire assembly is pushed axially of the housing 98 until the hub 116engages the left hand end 133 of the housing. The screw 108 may then betightened "to secure the guide 102 on the bracket 96. The wheel 138 isnext lowered into engagement with the chaser to grind the chamfer on it.While the grinding wheel 138 is rotating the chaser supporting element114 is rotated back-and-forth by the finger-wheel 120 so that theleading threads are ground transversely of the land or in other wordsfrom cutting edge 140 to the heel 142 thereof.

To limit the rotation of the element 114 in both directions there isprovided a stop pin 144 onthe left end of the housing 98. The pin 144extends into a slot 146 formed in the end of the hub 116 of supportingelement 114 and cooperates alternately with the opposite ends of theslot. The slot 146 is wide enough to permit grinding the full width ofthe land 86. Since the threads of the chaser 88 engage the threads ofthe guide 110 the oscillation of the supporting element 120 causes thechaser to move in the direction of the threads 110.

While the grinding of the chamfer 82 proceeds the grinding wheel 138 ismoved downwardly towards the ,chaser 88. It is arrested in its lowermostposition by an adjustable stop which may be similar to the stop 47d. Toprovide clearance for the wheel 138 the upper portion at the left handend of the guide 102 is cut away as at 148, Fig. 8. After the grindingoperation has been completed the chaser may be removed from the support114 and the next chaser 88a of the set may be mounted on and secured tothe support.

In Fig. 12 the chasers 88, 88a, 88b and 880 are shown in superposedrelation and the thread cutting teeth of each chaser are ofiset withrespect to those of the preceding chaser. Consequently, in order toproperly assemble the chaser 88a on the support and with respect to theguide 102 it is necessary to loosen the guide 102 so that it may bemoved slightly lengthwise of the housing 98 to have the thread 110thereof engage properly with the thread teeth 112 of the chaser. Afterthis has been accomplished the supporting element 114 is pushed againstthe face 133 of the housing 98 and the guide 102 is then secured to thebracket 96 by the screw 108. The grinding wheel 138 which has beenraised temporarily may now be lowered against the corner of the chaser88a to grind the chamfer thereon. During the grinding operation thechaser 88a is oscillated back-and-forth by the fingerwheel 120 ofsupporting element 114. The chasers 88b and 88c may be mounted on theelement 114 and ground in a manner similar to that described inconnection with the chasers 88 and 88a.

When all the chasers 88, 88a, 88b and 880 of the set have been groundthe chamfers on the chasers appear as in Figs. 11 and 12, where thesides of the complete chamfer faces 150 are parallel from the cuttingedge 14% to the heel 142 of each land. Their curvature extends about theaxis 152 (Fig. 9) of the set of chamfers, this axis 152 coinciding withthe axis of the guide 102 of Fig. 8. Because of the fact that eachchaser is caused to follow the threads of the guide 102 the amountground from each complete thread cutting tooth is equal throughout itslength. The grinding is efiected in the direction of the thread cuttingteeth on the land.

In the conventional chaser 88x (Fig. 13) each thread in the chamfer isprovided with a relief. Consequently more material is removed from thethread at the heel than at the cutting edge x to produce the relief.This results in tapered chamfer faces x. This curvature of each chamferface is eccentric with the axis 152 of the chaser 88x; the conventionalmethod of grinding the chamfer is known as eccentric grinding.

Briefly, the threading tool of my invention consists of means includinga land having thread cutting teeth extending cross-wise thereof, thecomplete leading threads being provided with concentric chamfer facesextending in the direction of the thread teeth.

My method and device for grinding a chamfer on the threading toolincludes means having the same size threads as the threading toolwhereby the tool is guided while it is being moved back-and-forth and incontact with a stationary grinding wheel to produce a chamfer on theleading threads of the threading tool.

In the form of the invention (Fig. 1) for grinding the chamfer on thetap 24 the tap is placed within the guide 36. The guide with tap thereinis then placed into the housing 32. After the finger-wheel 62 has beensecured to the tap shank the stop 52 is placed into one of the flutes 25and the tap is rotated until the cutting edge of land 22 engages thestop 52 which has been lowered into the flute 25. Thus the tap ispositioned with one of the lands up. The tap is then placed against theend stop 56. The screw 42 is next screwed down to secure the guide 36and the stop 52 is pulled up to its ineffective position out of the pathof the land 22. The grinding wheel 47 may then be lowered into contactwith the tap for grinding the chamfer. While the grinding is takingplace the tap is rotated back-and-forth a distance sufiicient to exposethe entire width of the land to the grinding wheel. As the grindingproceeds the wheel 47 is moved bodily closer to the tap, by the screw470 until the desired depth of the chamfer has been reached, the bodilymovement of the grinding wheel being arrested in this position by thestop screw 47d.

When the next land of the tap is to be ground the tap is given afraction of a rotation until the land previously ground passes the stop52. The stop is then lowered into the succeeding flute 25. The tap isturned farther until the cutting edge of the next land to be groundengages the stop. This causes the tap to advance axially in the guide 36due to the threads 20 of the tap engaging with the threads 46 of theguide 36. Consequently the tap shank 64 moves away from the end stop 56.The tap 24 is therefore repositioned on the bracket 28 and against theend stop 56 each time a land has been positioned for grinding.

To reposition the tap against the end stop 56 the screw 42 is backed 01fto loosen the guide 36 which guide is then moved axially of the housing32 until the tap shank 58 engages the end stop 56. The screw 42 issubsequently screwed down to secure the guide 36 and thus hold the guideand tap axially, in their readjusted position.

The stop 52 may then again be raised to its ineifective position so thatthe oscillation of the tap to grind the chamfer, on the newly positionedland, may be proceeded with. The above described operations are repeatedfor the positioning and grinding of each land.

It will be understood because of the repositioning of the tap againstthe end stop 56 before grinding each land that the chamfers on all ofthe lands are alike. This causes the lands to cut uniformly and at thesame time when the tap enters a hole to be tapped.

In the form of the invention shown in Fig. 8 for grinding the chamfersindividually on the various chasers of enemas 7 a set, the guide 102 isplaced in the bore 100. of the housing 98 with its left hand endprotruding from the housing. The supporting element 114 is then placedon the bearing portion 118 of the guide 102. The first one, 88, of theset of chasers is then placed against the shoulder 126 and mounted onthe supporting element 114. The chaser 88 and guide 102 are adjusted sothat a few of its threads 112 engage properly with a correspondingnumber of those of the guide 162. The assembly, including the guide 102,the supporting element 114 and chaser 88, is then pushed axially of thehousing 98 until the face of the hub 116 engages the face 133 of thehousing. The guide 162 is then secured in position axially of thehousing 98 by the screw 108. The chaser may then be oscillated relativeto the grinding wheel 138 through the medium of the finger-wheel 120While grinding the chamfer. The grinding wheel 138 may be thin with arounded grinding edge.

After the chamfer is completely ground the chaser 38 is removed and thesecond chaser 88a is mounted on the supporting element 114. The screw108 is then backed oil? to loosen the guide so that it may be movedaxially of the housing until the threads 110 engage properly with thoseof the chaser 38a. The chaser 88 may then be secured to the supportingelement 114 by the screw 130. Subsequently the assembly, including thechaser 88a, element 114 and guide 182, is placed with the hub 116engaging the face 133. After securing the guide by thumb screw 108 thesupport 114 may be oscillated by its fingerwheel to move the chaserback-and-forth relatively to the grinding wheel. The supporting element114, it will be remembered, is arrested in its extreme position by thestop 144.

Although the invention is shown and described in connection with a toolsuch as a tap for cutting internal threads and in connection with a toolincluding a set of chasers for cutting an external thread, it should beunderstood that the invention may also readily be embodied in otherthreading tools and machines for grinding the chamfers on the landsthereof. Such tools may include dies for cutting external threads and aset of Chasers for cutting internal threads.

While certain preferred embodiments of the invention have beendescribed, it will be understood that 03. tain parts may be used withothers and that changes in the form, arrangements, proportions, sizesand details thereof may be made without departing from the scope of theinvention as defined in the appended claims.

I claim:

2. In a machine for grinding a chamfer on the land of a tap havingthread cutting teeth, the combination of a grinding wheel, a stationarybracket, a threaded element on said bracket to engage the teeth of thetap, means associated with said tag to rotate the latter back-and-forthabout its axis and relatively to the grinding wheel, means to engageeach land of the tap individually to position the lands successively atthe wheel, means to actuate the positioning means to an inactiveposition, and means to hold the positioning means in its inactiveposition.

2. In a machine for grinding a chamfer on the lands of a tap. each landhaving thread cut-ting teeth, the combination of a grinding wheel, astationary bracket, a thread on said bracket to engage the teeth of thetap, means associated with said tap to rotate the latter back-andrfor'thabout its axis and relatively to the grinding =wheel, means to engageeach land of the tap individually to position the lands successiveiy ina grinding position, means to assist in repositioning the tap on thebracket after the grinding of each land, and means for securing the :tap;to said bracket after each reposition operation thereof.

3. I n a machine for grinding achamferon the lands of a tap, each landhaving thread cutting teeth, the combinan o a e d a h e a station rybracket, a housi on said bracket, sai .liousi gihavi gaa ax a ceive thetap, a thread in said bore to engage the teeth t the p. m ans ass stat tsaid t n o rotate t e latter batflt and forth about its axis relativelyto the grinding wheel, means to engage each land of the tap individually to position the lands successively in a grinding posit on, means toassist in repositioning the tap with the housing axially each time aland is positioned for grinding, and means for securing the housing tosaid bracket after each repositioning operation of the tap and itshousing.

In a machin f r g nd hem st On h d of a tap, each land having threadcutting teeth, the combination of a grinding wheel, a stationarybracket, a housing on said bracket, said housing having an axial bore toreceive the tap, a thread in saidbore to engage the teeth of the tap,means associated with said tap to rotate the latter c ria-f rth bo t iax s a rel i tq he grinding wheel, means to engage each land of the tapd ally t p siti n th ands su ces i in inding position, means to assistin repositioning the tap with h h u in axially e me a l n s sit ans rgri in s o n ha n a sl n i ut face a extending axially ofsaid housing,and a screw threaded through said bracket, and extending into said slotto prevent circumferential displacement of the housing and to secure thehousing axially of said bracket after each re: positioning operation ofsaid tap and said housing.

5. In a machine for grinding a chamfer on the land of a chaser havingthread cutting teeth, the combination of a grinding wheel, a stationarybracket having a bore therein, a shaft in said bore, said shaft having aportion extending beyond said bracket, threads on the extending portionof said shaft, a supporting element revolubly mounted on the shaftportion, means to demountably attach the chaser to said supportingelement, means to lo: cate the chaser on the supporting element so thatits teeth engage the thread on said shaft, and means to rotate saidsupporting element back-and-forth to corresponding- 1y actuate thechaser relatively to the grinding wheel.

6. In a machine for grinding a chamfer on the land of a chaser havingthread cutting teeth, the combination of a grinding wheel, a stationarybracket, means including a stationary threaded element on the bracket,said threaded element having a curvature about an axis, the threadedelement having thread sections, a supporting element revolubly mountedon said threaded element, means to demountably attach the chaser to saidsupporting element, means for locating the chaser to said supportingelement with its teeth engaging the thread sections of said threadedelement, and ineans to rotate the supporting element baclg-and-forthabout the axis of the threaded element to correspondingly actuate thechaser relatively to t in in whee 7.- n a m chin for ndi a s m e l nd ofa chaser having thread cutting teeth, the combination of a grindingwheel, a stationary bracket having a bore therein, ,a shaft in saidbore, said shaft having a portion extending beyond said bracket, threadsections on the free end of the shaft portion, a supporting elementrevolubly mounted on the shaft portion, means to demountably attach thechaser to said supporting element with the teeth in engagement with thethread sections, a face on said bracket against which the supportingelement bears, means to prevent rotary displacement of said shaft, meansto secure said shaft against accident axial displacement, n me n o rotae the su aq b cknd-t t t 9;- espondinslv aerat th chas r l t e t9 hegrind n wheel.

8. in a machine for grinding a tap, the tap having a plurality of lands,each land having adjacent thread cutting teeth extending crosswisethereof, the combination of a rotatable grinding wheel arranged to grinda chamfer on each land individually at the leading end of the tap, meansto rotate the tap back-andforth about its axis, .-.ea s to mandat witheach lead t qs tian the t strguintereat ally so that th s en ma be a u das each land successively by rotating the tap back-aud-forth while incontact with the wheel, means to guide the tap in the direction of thepitch of its thread cutting teeth during its back-and-forth rotativemovement, and means to facilitate adjustment of the guiding means withthe tap axially of the tap after the grinding operation of each land sothat the next succeeding land may occupy the same initial positionoccupied by the previously ground land.

9. The method of forming a concentric chamfer extending over a pluralityof helically arranged thread cutting teeth on the land of a threadingtool, which includes presenting the crest portions of some of the teethsimultaneously to the concentric face of a rotating grinding wheel,reciprocating the tool about its axis, guiding the teeth in theirhelical direction from the cutting edges to the heels of the teeth whilethe tool is in contact with the grinding wheel so that the chamfer faceformed on each complete thread tooth is concentric and of uniform width,having the active portion of the grinding surface of the wheel arrangedoblique to crests of some of the teeth so that each succeeding chamferface on each succeeding tooth of the land after grinding is narrowerthan the chamfer face on its adjacent preceding tooth.

10. In a machine for grinding a concentric chamfer on the land of athread cutting tool having thread cutting teeth, the combination of agrinding wheel having a concentric grinding surface, a stationarybracket to support the tool, means to rotate the tool back and forthrelatively to the grinding surface of the wheel during rotation of thelatter to remove the crests of some of the teeth simultaneously, meansto cause the tool to move in the direction of the slant from front toback of the thread cutting teeth while the tool is being rotated, andmeans to present the grinding surface of the wheel obliquely to thecrests of a plurality of said teeth.

11. In a machine for grinding a concentric chamfer on the land or a taphaving thread cutting teeth, the combination of a grinding wheel, astationary bracket, a housing on said bracket and at one side of thewheel, said housing having an axial bore to support the tap with itsfree end exposed, a thread in said bore to engage the thread cuttingteeth of the tap to guide the tap in a helical path, and a finger piecedemountably attached to said tap to rotate the latter back-annd-forthabout its axis while its free end is in contact with the grinding wheel.

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